During manufacture of integrated circuit (IC) devices, dielectric layers are often used to aid the fabrication process. For example, to protect active areas of a silicon substrate during formation of oxide isolation regions (e.g., field oxide regions), a silicon nitride layer will be formed over the active areas of the substrate. While forming the oxide isolation regions, the surface of the silicon nitride layer becomes oxidized. After aiding the fabrication process, the oxidized silicon nitride layer must be removed.
Several processes are known to remove oxidized silicon nitride layers from IC devices. In one removal process, described in U.S. Pat. No. 3,709,749 and incorporated herein by reference, a substrate containing the oxidized silicon nitride layer is dipped in high-temperature (100° C.) water. Other removal processes use phosphoric acid (H3PO4). See, for example, W. van Gelder et al., Journal of the Electrochemical Society: SOLID STATE SCIENCE, Vol. 114, No. 8, pp. 869-72 (August 1967); U.S. Pat. No. 4,092,211; and K. Sato et al., Detailed Study of Silicon-Nitride-Etching Mechanism by Phosphoric Acid for Advanced ULSI Processing (Abstract), Tohoku University (date unknown), the disclosure of each of which is hereby incorporated herein by reference. At low temperatures, phosphoric acid is unable to significantly etch the silicon nitride because of its inability to appreciably attack the silicon oxide. Higher temperatures speed up the attack of the silicon oxide, but decrease the etch rate of the silicon nitride. As a result, it has been difficult to etch an oxidized silicon nitride structure using phosphoric acid.
In an attempt to increase the etch rate of silicon oxide at low temperatures, fluoroboric acid (also referred to herein as “fluoboric acid”) has been combined with phosphoric acid as described in U.S. Pat. No. 3,859,222, incorporated herein by reference. However, adding fluoroboric acid has not significantly improved the ability of phosphoric acid to etch the oxidized silicon nitride structure without also attacking and degrading the oxide isolation regions.
Hydrofluoric (HF) acid has also been employed to etch oxidized silicon nitride structures. Unfortunately, the selectivity of HF acid is negative or, in other words, HF acid severely etches silicon oxide to the extent of removing silicon oxide at a rate faster than silicon nitride, producing unfavorable geometry for further IC device processing. When a field oxide region is present, the negative etch selectivity removes large amounts of the field oxide region, thus impairing the ability of the field oxide to act as an isolating region.
Another removal process uses HF acid in a first step and phosphoric acid in a second step to etch oxidized silicon nitride structures. See U.S. Pat. No. 3,657,030, incorporated herein by reference. The HF acid etches off enough of the oxide surface to enable the phosphoric acid to attack the silicon nitride. Too little removal by the HF acid prevents the phosphoric acid from attacking the silicon nitride, while too much removal by the HF acid unduly depletes the oxide isolation regions. Etching with HF acid followed by phosphoric acid, however, also increases the materials used as each HF and H3PO4 etching step is followed by a rinsing and drying step, thus, increasing the complexity and cost of the fabrication process.